In recent years, light-emitting diodes (LED) are rapidly spreading. Light-emitting devices such as LED in widespread use for a large display such as TV and PC, a touch panel, an information terminal such as cellular phone and smartphone, and an illumination system, etc. are making progress toward higher luminance. To keep pace with this progress, a high performance is required also of an encapsulant used for these devices.
So far, a method of using a novolak-type epoxy resin, an alicyclic epoxy resin or an acrylic resin as an encapsulant and curing the resin with an acid anhydride or a polymerization initiator to resin-encapsulate LED, etc. has been attempted (see, for example, Patent Documents 1 and 2). Among others, epoxy resin has an excellent ability to protect a metal component or a device from external environment and therefore has been widely used as an encapsulant for an electronic material.
However, due to exposure to heat from a light-emitting element or an ultraviolet ray, many epoxy resins are involved in the progress of deterioration and cannot maintain high transparency, resulting in coloration. Accordingly, in the field requiring high transparency, such as LED, utilization of an epoxy resin is avoided.
A dimethylsiloxane-based silicone-type resin or random siloxane-based curable resin composition is used as an encapsulant having high transparency (see, for example, Patent Documents 3 and 4). Such a curable resin composition is cured by condensation of silanol groups with each other or by hydrosilylation reaction of a carbon-carbon double bond with an SiH group, using a platinum catalyst.
These curable resin compositions containing siloxane as a main component can maintain high transparency over a long period of time even when exposed to heat or light and therefore are widely used as an encapsulant for LED, etc. However, further improvements are required of the curable resin composition, because, for example, the adherence to a base material is poor, the refractive index of the resin is lower than the value required by an encapsulant, or there is a problem in satisfying both tack property on the cured product surface and crack resistance during curing.
In addition, the silicone-based encapsulation resin is poor in the sulfur gas resistance and water vapor barrier property and therefore suffers from a problem that a metal component is discolored to decrease the luminance due to permeation of a corrosive gas through the encapsulation resin. To solve this problem, for example, an attempt is made to improve the permeability of sulfuric gas or water vapor by increasing the crosslinking density of a silicone resin or a siloxane resin (see, for example, Patent Documents 5 and 6). On the other hand, a silicone resin is grafted to a silica fine particle with an attempt to achieve higher gas barrier property than that of conventional silicone resins (for example, Patent Document 7).
However, their performance is not sufficient in terms of gas barrier property, as compared with conventional epoxy resins. Furthermore, it has been discovered that when a dimethyl silicone-based resin is exposed to heat for a long period of time, the resin is gradually decomposed to reduce the thickness and the gas barrier property and mechanical properties are deteriorated with aging. An attempt is made to improve thermophysical properties and mechanical properties by introducing a tough framework into the side chain of a silicone resin, but this technique is not completely complementary to maintaining physical properties for a long period time, because after all, the dimethyl silicone moiety is deteriorated and a corrosive gas permeates the moiety.
Under these circumstances, an encapsulation resin having both high gas barrier performance and mechanical properties comparable to an epoxy resin while maintaining high transparency comparable to a silicone resin is demanded.